Pack of 12 sold seperately Bobbins are a replacement part for all Halkey Roberts type automatic inflators. They dissolve when immersed in water and cause the CO2 cylinder to inflate the PFD. All Mustangs automatic inflatable PFDs come with cylinders and bobbins. Annual maintenance and inspection of your PFD is recommend. For recreational use bobbins should be replaced every three years to avoid inadvertent inflation.